1. Field of the Invention
The present invention relates to a microwave oven door, and more particularly, to an apparatus for shielding electromagnetic wave that compensates for the length of slots formed in an end of an oven door in order to improve the shielding ability capable of preventing the leakage of electromagnetic wave from the cavity.
2. Description of the Related Art
An electric oven generally uses an electric heater as a heat source for generating heat to cook a food loaded into a cooking chamber, and may be auxiliary provided with another heat source. For this purpose, for example, a magnetron is installed in the electrical oven in order to provide electromagnetic wave as the additional heat source.
The operation of a typical electric oven will be described as follows:
In an illustrative electric oven as shown in FIG. 1, a user opens an oven door 3 by pulling a door handle 4 with a hand, loads a food to be cooked into a cavity 2 within an oven housing, closes the oven door 3 to seal the cavity 2, and then operates the electric oven to cook the food.
The user opens/closes the cavity 2 by using the door handle 4 mounted on a top portion of the oven door 3. In this case, a hinge (not shown) connecting a lower end of the housing 1 with a lower end of the door 3 allows the door 3 to be pivoted forward/backward so that the cavity 2 is opened/closed.
Heat generated from a lower heater 7, which is mounted between the bottom of the cavity 2 and the housing 1, is transmitted to the bottom of the cavity 2. Then, the heat is transmitted to the food to be cooked through the air within the cavity 2 and a tray loaded with the food.
Further, heat generated from an upper heater 9, which is mounted above the cavity 2, is transmitted to the food through transmission and convection, and a convection fan 13 is actuated to transmit heat generated from a convection heater 11 to the food in the form of hot wind through a number of through holes 13a perforated in the rear side of the cavity 2. In this way, the food loaded on the tray is cooked.
Electromagnetic wave oscillated from a magnetron 15, which is installed in an upper portion of the cavity 2, is directed into the cavity 2 through a waveguide 16 placed above the cavity 2 to function as a heat source of the food to be cooked. The magnetron 15 can be optionally used by the user to cook the food. A cooling fan 17 serves to cool electric components including the magnetron 15, and an oven lamp 18 is configured to illuminate inside a cooking chamber defined by the cavity 2.
Since the typical electric oven cooks the food by using electromagnetic wave generated from the magnetron as described above, it is necessary for the electric oven to prevent the leak of radio frequency radiation. When the door is closed, the electric oven has a uniform gap between the cavity and the door, which forms a slot waveguide allowing the leakage of electromagnetic wave energy generated from the magnetron. In order to prevent the leakage of electromagnetic wave energy, the electric oven is provided with an electromagnetic wave absorbent or a filter around the door or a cavity opening. The filter is generally provided with a choke of a ¼ wavelength dispersion parameter around the door, in which the choke is coupled with the cavity opening.
An apparatus for heating dielectrics by using radio frequency (such as an microwave oven, electric oven, OTR and the like) as described above is configured to trap electromagnetic wave with a cavity 110 and a door 111 as shown in FIG. 2. A filter is installed in a contact region between the cavity 110 and the door 111 in order to prevent the leakage of electromagnetic wave to the outside.
The contact region may have various structures as shown in FIGS. 3 and 4 according to oven types. FIG. 3 illustrates a contact region between a cavity and a door in an electric oven, and FIG. 4 illustrates a contact region between a cavity and a door in a microwave oven.
FIG. 3 illustrates an L-shaped inner end 122 of an oven door 121 coupled with a front portion of a cavity 120 and a choke structure 162 applied to the inner end 122. FIG. 4 illustrates an inner end 132 of an oven door 131 coupled with a front plate 134 of a cavity 130 and a choke structure 142 applied to the inner end 132. In the above types of contact regions, the choke structures 162 and 142 for interrupting the outer leakage of electromagnetic wave are provided in the inner ends 122 and 132 of the oven doors 121 and 131 and the cavities 120 and 130, respectively.
In the meantime, filters as shown in FIGS. 5 and 6 may be selectively applied according to types of the oven shown in FIG. 4.
Referring to FIG. 5, a U-shaped choke structure 142 is formed in an inner end of a door (or door frame) 131. The choke structure 142 is bent into three sections including a choke base 143, a choke inner side portion 144 and a choke top 145. A plurality of L-shaped slots 146 are formed in the choke top 145 and the choke inner side portion 144 at a predetermined interval.
Each of the slots 146 is extended along a first length L1 corresponding to the entire width of the choke top 145 and a second length L2 corresponding to a greater portion of the entire width of the choke inner side portion 144. The slots 146 have a uniform width W, and the choke top 145 is opposed to the cavity.
This choke structure 142 functions to shield the leakage of electromagnetic wave from the cavity.
Referring to FIG. 6, a choke structure 152 is formed in an inner end of a door 131. The choke structure 152 is bent into U-shaped three sections including a choke base 153, a choke inner side portion 154 and choke top 155. A plurality of L-shaped slots 146 are formed in the choke inner side portion 154 and the choke top 155 at a predetermined interval.
Each of the slots 156 is extended along a first length L11 corresponding to the entire width of the choke top 155 and a second length L12 corresponding to a greater portion of the entire width of the choke inner side portion 154. The each slot 156 has a first width W11 in the choke top 155 and a second width W12 in the choke inner side portion 154, in which the second width W12 is larger than the first width W11.
FIG. 7 is a graph illustrating shielding properties of the filters shown in FIGS. 5 and 6, in which though incidence angles have a diverse range of from 0° to 90°, only three different incidence angles are exemplified for the convenience of description.
As shown in FIG. 7, when radio frequencies of different incidence angles I1, I2 and I3 are introduced into the choke structures as shown in FIGS. 5 and 6, respectively, the choke structure shows a shielding property that the optimum shielding frequency is lowered from fo to fo′ as the incidence angle increases. That is, it is known that when the incidence angle I3 is 16.7°, the optimum shielding frequency is fo, and when the incidence angle I1 is 39.9°, the optimum shielding frequency is fo′. The shielding ability is limited to a specific single frequency. This feature represents that the optimum shielding frequency fo is lowered in reverse proportion to the variation of the incidence angles that increases in the order of 16.7, 23.4 and 36.9 degrees.
FIG. 8 illustrates another example of the conventional filter, which is generally adopted in the electric oven as shown in FIG. 3.
Referring to FIG. 8, a choke structure 162 formed in an inner end of a door 121 is bent into three sections including a choke base 163, a choke inner side portion 164 and a choke top 165 which has a narrow gap with cavity side face. Slots 166 are formed only in the choke top 165 at a predetermined interval. That is, rather than being formed in the entire width L21 of the choke top 165, each of the slots 166 is formed along a first length L22 corresponding to a greater portion of the entire width L21 of the choke top 166 except for a second length L23 extended from a bent of the choke top 165. The length of the slot L22 is obtained by subtracting the second length L23 from the entire width L21 of the choke top 166.
This as a consequence induces electromagnetic wave introduced into the cavity from the magnetron to resonate into specific modes, which in turn determine an incidence angle into the oven door. Since a simple nλ/4 (n=1,3,5, . . . ) choke structure can rarely shield electromagnetic waves having various incidence angles, a plurality of slots 156 and 166 are provided at a predetermined interval. The interval of the slots 156 and 166 is designed to effectively shield electromagnetic waves of any incidence angles.
Incidence angle dependency is regarded as one of important factors for determining the performance of the filter. Since electromagnetic waves generated inside the cavity are distributed into a very complicate mode, they are directed toward the door at various angles ranging from 0 to 90 degrees. Therefore, an excellent filter is required to properly shield the electromagnetic waves directed to the door regardless the incidence angles of the electromagnetic waves. That is, the excellent filter is required not to have incidence angle dependency. However, there is a problem in that existing filters basically have incidence angle dependency. Simulation results of the existing filters are illustrated in FIGS. 7 and 9.
FIG. 7 is a graph illustrating shielding properties of the filter shown in FIG. 6 and FIG. 9 is a graph illustrating shielding properties of the filter shown in FIG. 8.
Referring to FIG. 9, when a radio frequency is incident into the choke structure at three different incident angles I11 (15.6°), I12 (21.7°) and I13 (35.6°) as shown in FIG. 8, the choke structure shows a shielding property that the optimum shielding frequency increases as the incidence angle increases. That is, it is known that when the incidence angle I11 is 15.6°, the optimum shielding frequency is fo, and when the incidence angle I13 is 35.6°, the optimum shielding frequency is fo′.
since the microwave oven or electric oven is operated in a single frequency, and in the conventional filters, the optimum shielding frequency is changed from fo to fo′ with respect to some of total incidence angles ranging from 0 to 90 degrees as can be seen in Figs. these filters have limited shielding properties for electromagnetic waves of various incidence angles. Because the shielding ability of a filter is evaluated from its worst shielding level, this necessarily limits the shielding ability of the conventional filters.